1. Field of the Invention
The present invention relates to a lens unit and an electronic camera using it. More particularly, this invention is concerned with a lens unit for driving a collapsible zoom lens using a stepping motor, and an electronic camera having the lens unit. In particular, this invention is concerned with driving and controlling of a lens barrel of the electronic camera.
2. Description of the Related Art
In recent years, cameras for achieving photography using photographic film or electronic cameras (hereinafter these types of cameras are generically called cameras) are demanded to have a compact camera body for convenience in portable use. The electronic cameras produce an electrical image signal using an imaging device or the like, and record the image signal on a recording medium or the like.
Moreover, the cameras are requested to offer more advanced functions, or specifically, to include a high-precision automatic focusing (AF) unit, and a high-power zoom lens that is used as an imaging lens.
The conventional cameras have been devised from various aspects in an effort to realize both a compact camera body and advanced functions at the same time. For example, cameras having a collapsible zoom lens unit or the like have become prevalent. Specifically, when such a camera is unused, an imaging lens barrel accommodating a plurality of lens frames that hold an imaging lens (or a zoom lens barrel when the imaging lens is a zoom lens) is stowed in a camera body. The camera thus becomes compact enough to be portable. When the camera is used, the lens barrel is thrust in front of the camera body. The plurality of lens frames in the lens barrel are moved to predetermined positions, whereby a power variation action (zooming action) or an automatic focusing (AF) action is executed.
In the collapsible zoom lens unit or the like, the lens frames are moved over two movable intervals, that is, a stowage interval and a zoom interval. Herein, the stowage interval is a movable interval between a position of stowage at which the lens frames are stowed in the camera body and a ready-to-image position at which the lens frames are thrust from the front surface of the camera body and imaging is enabled. The zoom interval is a movable interval over which the lens frames move with the ready-to-image position as a start point during execution of an imaging action, that is, an interval over which a predetermined lens frame is moved for achieving a zooming action.
The movable intervals moved by the lens frames are, as mentioned above, intervals moved thereby for achieving two different actions. Namely, the movable intervals include the zoom interval (or zooming area) moved for achieving a zooming action and the stowage interval moved for stowing the lens frames in the camera body. In this case, requirements to be met for driving the lens frames are different between the intervals.
Normally, movements made over the zoom interval by the lens frames are movements intended mainly for achieving an imaging action. Higher precision is therefore requested for the zoom interval. By contrast, high precision is not requested for the stowage interval, but a quicker action is requested. The lens frames (lens barrel) must be moved to their stowage positions by making fast and reliable movements. It is therefore desired to drive and control the lens frames under the conditions for driving that are optimal for each movable interval.
From this viewpoint, a lens drive unit disclosed in, for example, Japanese Patent No. 2702474 is designed to change a driving speed, at which a zoom lens is driven, between the zoom interval and any other interval. Namely, over the zoom interval, the driving speed at which the zoom lens is driven is controlled so that the zoom lens is driven at a low speed. Over a non-zoom interval, the driving speed at which the zoom lens is driven is controlled so that the zoom lens will be driven at a high speed.
According to the lens drive unit, the zoom lens is moved at a high speed over the non zoom interval. This makes it feasible to shorten the time required for changing the state of the unit from a movable state to a stowed state.
As mentioned above, according to the means disclosed in the Japanese Patent No. 2702474, it is noted that the driving speed at which the zoom lens is requested to be driven is different between the zoom interval and the other movable interval. Control is therefore given so that the conditions for driving the zoom lens will be changed between the zoom interval and the other movable interval.
However, a normal collapsible zoom lens unit is requested to meet the conditions for driving described below in addition to the driving speed at which the zoom lens is driven.
Normally, a means realized with a devised mechanism including a cam or the like for moving lens frames is used to move the lens frames at a relatively high speed over a stowage interval over which the lens frames are moved to be stored. Moreover, an unexpected extraneous force may be applied to a lens barrel holding the lens frames. For driving and controlling the lens barrel over the stowage interval over which the lens barrel is moved to be stowed, a sufficient torque is needed for driving.
Moreover, it is indispensable to move the lens barrel in finer steps over a zoom interval. This is intended to adjust the power of the zoom lens highly precisely. For reliably positioning a predetermined lens frame at a predetermined position, driving must be controlled highly precisely.
Furthermore, for example, in an electronic camera or the like, an electric circuit or the like relevant to an imaging system that consumes a relatively large amount of power for execution of an imaging action must be operated. Power to be consumed by any system of electric circuits other than the imaging system must therefore be minimized during execution of the imaging action.
A zooming action requires a relatively large amount of power and is frequently utilized during execution of the imaging action. Once a driving current needed for achieving the zooming action can be minimized, great power saving is realized. Minimization of the driving current needed for achieving the zooming action during execution of the imaging action is what has especially been demanded in recent years.
By the way, an unexpected extraneous force may be unintentionally applied to the lens barrel holding the lens frames, for example, as shown in the illustrative drawing of FIG. 9, an extraneous force F may act on a front lens frame 102 during imaging. This causes the lens frame 102 to move inward a camera body 101. The front lens frame 102 may be displaced from a predefined position at which it should be placed.
Conventionally, the lens frame or the driving cam is provided with an encoder for detecting a position. A lens barrel driving method employing a so-called closed loop realized by combining the encoder and a driving DC motor is adopted in order to solve the trouble of the foregoing displacement caused by the extraneous force P. According to this method, even when the position of a lens frame is changed due to the extraneous force or the like, the change in position can be detected by the encoder and can therefore be corrected.
Examples of lens barrels coping with an abnormal movement of a lens frame are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 6-347683, 7-218807, and 9-230215. These technical means utilize an output of an encoder for detecting the position of a lens frame so as to detect an abnormal movement.
As for a method of driving lens frames, as already known, a driving method utilizing a stepping motor, which is controlled with driving pulses, as a driving source is advantageous over the method using the DC motor and encoder in combination in terms of costs and an occupied space. For example, Japanese Unexamined Patent Publication No. 5-188267 discloses a means using a stepping motor as a source for driving lens frames and a photosensor as a detecting means for detecting a reference position (home position).
The conditions for driving including a driving speed at which lens frames or the like are driven have a close relation to the driving source such as a motor. For controlling the driving source in strict conformity with the conditions for driving, the driving source must be limited to some type. Normally, as the driving source for driving lens frames in a camera or the like, a stepping motor obviating the necessity of an encoder and other members has been widely adopted in the past.
Now, general methods of driving a stepping motor will be described briefly. Normal methods of driving a stepping motor include a single-phase excitation driving method shown in FIG. 10, a two-phase excitation driving method shown in FIG. 11, a single/two-phase excitation driving method shown in FIG. 12, a micro-step driving (not shown) method, and other various driving methods.
The single-phase excitation driving method is a driving form of alternately conducting electricity to phase-A and phase-B coils as shown in FIG. 10. The coils can be driven with low power consumption, while a driving torque is relatively low.
Moreover, according to the two-phase excitation method, a rotor moves so that the magnetic poles thereof will be opposed to each other between two adjoining magnetic poles of a stator. A magnitude of rotation, by which the stepping motor rotates, caused by one change of the magnetic poles of the stator is identical to that according to the single-phase excitation method. The two-phase excitation method is a driving form for conducting electricity to the phase-A and phase-B coils simultaneously as shown in FIG. 11. In the two-phase excitation method, the stator must be excited all the time in order to keep the stepping motor stopped. The two-phase excitation method is characterized in that much more power consumption is needed compared with the single-phase excitation method but the stepping motor can be driven with a higher driving torque than that according to the single-phase excitation method. Consequently, the two-phase excitation method realizes faster movements.
According to the single/two-phase excitation method, single-phase excitation and two-phase excitation are repeated. For example, assume that the rotor starts rotating at a position at which it is opposed to one magnetic pole of the stator. With the next change of the magnetic poles of the stator, the rotor moves to be interposed between the two adjoining magnetic poles of the stator. With a subsequent change of the magnetic poles, the rotor moves to a position at which it is opposed to the other adjoining magnetic pole of the stator. A magnitude of rotation derived from one change of the magnetic poles of the stator is equivalent to a half of the magnitude of rotation provided according to the single-phase or two-phase excitation method. The magnitude of rotation derived from one change of the magnetic poles can be made smaller than that provided according to the single-phase or two-phase excitation method. The single/two-phase excitation method can achieve finer driving and ensure high positioning precision. Moreover, the single/two-phase excitation method has the merits of few vibrations and a low-pitch noise.
The micro-step driving method can electrically realize a microscopic step angle. The micro-step driving method is characterized in that it can ensure higher positioning precision than the single-/two-phase excitation method and contribute to realization of few vibrations and a low-pitch noise.
When the stepping motor is utilized, the driving method for driving the stepping motor should be changed according to the required conditions for driving. Thus, lens frames can be moved more efficiently. This is very convenient.
As far as a zoom lens barrel of an inner focus type is concerned, the positional relationship between zoom lenses and focus lenses is the greatest issue that must be settled to prevent displacement of a front lens frame caused by an extraneous force. According to a focusing method widely adopted for conventional silver film cameras, the positions of the focus lenses are determined based on the positions of the zoom lenses and measured distance information. It is impermissible that the front lens frame is displaced from its predefined position. According to the conventional means, it is indispensable to use the encoder to detect the position of a lens frame so that the lens frame can be controlled to lie at its predefined position all the time.
Moreover, conventional electronic cameras or video movies generally adopt a so-called contrast focusing method. According to the contrast focusing method, a high-frequency component of a signal acquired by an imaging device is detected in order to attain an in-focus state. At this time, distance information is unused. When this focusing means is employed, the positional relationship between the zoom lenses and focus lenses is not requested to be strictly accurate.
However, when a conventional electronic camera or the like adopts the contrast focusing method, as long as a displacement of the front lens frame caused by an extraneous force is limited, there is no obstacle to an imaging action. The necessity of the encoder is therefore obviated. However, when the stepping motor is used as a driving source for moving lens frames, the magnitude of displacement of the front lens frame that is of a certain level or more cannot be corrected unless the lens frames are returned to their reference positions (home positions). When a photographer uses a camera to execute an imaging action, the front lens frame out of all the lens frames included in the camera may be displaced to a larger extent. In this case, the photographer may continue imaging without being aware of the fact that the front lens frame is displaced. This results in certain problems as described below.
The distance between the zoom lenses and focus lenses becomes different from the predefined one. The focus lenses that are moved during execution of a focusing action may collide with the adjoining zoom lenses or the like.
Moreover, after the focusing action is completed, the distance to an object may be calculated based on the relationship between the positions of the zoom lenses and those of the focus lenses. Information concerning the distance to an object may be utilized as camera control information used to control a strobe unit or the like. In this case, if the displacement is very large, an error in camera control information becomes very large. This may lead to impairment of image quality.
Because of these troubles, as long as a zoom lens barrel having a front lens frame capable of advancing or withdrawing freely is adapted to a conventional electronic camera or the like, it is difficult to design a barrel driving structure, which uses a stepping motor as a driving source, without an encoder.
The present invention has been devised in an effort to overcome the above problems.